1. Field of the Invention
The present invention relates to foamed thermoplastic resin moldings which can be used, for example, as food product containers which can contain retort food products, etc., household electrical appliance parts and automotive interior parts, and to a process for producing the same.
2. Description of the Related Art
Foamed thermoplastic resin moldings prepared by shaping foamed thermoplastic resin sheets as a raw material resin sheet have been used in a wide range of application fields such as food product containers, household electrical appliance parts and automotive interior parts through making the excellent characteristics, such as light weight and heat insulation properties, which the moldings have. The foamed thermoplastic resin moldings used in such applications are naturally required to have certain rigidity according to their individual applications.
In general, light weight, heat insulation property and soft feeling, however, are characteristics conflicting with rigidity. In other words, to enhance the light weight, heat insulation property and soft feeling, it is necessary to make the expansion ratio of the foamed thermoplastic resin sheet to be used as the raw material resin sheet high. However, the fact is that the moldings using such highly expanded raw material resin sheets have good characteristics such as light weight, but the rigidity is deteriorated, resulting in unsatisfactory products.
As a technique for solving such problems, the invention disclosed in Japanese Unexamined Patent Publication JP-A-8-11149 is known. The technique disclosed in this publication relates to a foamed resin sheet having an expansion ratio of not less than 1.1 times and a specific gravity of not greater than 0.8 wherein a foaming condition in its one side is different from that in the other side, and to a container obtained by using the resin sheet.
Although a method for producing the foamed resin sheet is disclosed in the above-mentioned Japanese Unexamined Patent Publication JP-A-8-11149, there is no concrete description on a molding and moldings prepared by shaping according to the conventional technique for producing foamed resin containers must be unsatisfactory in rigidity.
The objects of the present invention are to provide foamed thermoplastic resin moldings which are lightweight, have heat insulation properties and soft feeling and also possess rigidity, and to provide a process for producing such moldings.
To attain the above objects, the present inventors investigated foamed thermoplastic resin moldings and a process for their production. As a result, after finding that not making the whole molding a uniform foam but imparting high heat insulation property and soft feeling to only a necessary part of the molding resulting in a foamed thermoplastic resin molding excellent in rigidity and also highly lightweight, the inventors have reached the present invention.
The foamed thermoplastic resin molding of the present invention is one prepared by shaping a foamed thermoplastic resin sheet, the molding comprising a general part and a thick part wherein these parts differ from each other in thickness wherein the mean expansion ratio of the thick part is not less than 1.2 times the mean expansion ratio of the general part and wherein the maximum thickness of the thick part is not less than 1.5 times the thickness of the general part.
Forming the general and thick parts rather than constituting the whole body of a uniform foam has resulted in the improvement of rigidity without accompanying the damage of light weight, soft feeling and heat insulation property.
The thick part is located at a position where soft feeling, heat insulation property or rigidity is required in consideration of the usage conditions of the molding.
Moreover, although the molding of the present invention is constituted essentially of such a general and thick parts, it may further have an auxiliary part which is non-foamed or of an expansion ratio lower than that of the general part, such as flanges of containers or the like, as needed.
It is preferable that the mean expansion ratio of the thick part is not less than 1.5 times and not greater than 5 times the mean expansion ratio of the general part since moldings having the greater balance of soft feeling, heat insulation property and rigidity can be prepared
The foamed thermoplastic resin sheet to be used as the raw material resin sheet of the molding is preferably a foamed thermoplastic resin sheet having an expansion ratio of not greater than 5 times since moldings having the aforementioned mean expansion ratios can be certainly prepared.
The foamed thermoplastic resin sheet is preferably a foamed polyolefin-based resin sheet since moldings having required strength can be prepared at low cost.
The foamed thermoplastic resin molding of the present invention preferably further comprises a skin material layer laminated as its surface.
This results in moldings excellent in feeling to the touch and in external appearance which are particularly suitable for automotive parts applications.
The present invention provides a process for producing the aforementioned foamed thermoplastic resin molding, the process using a mold comprising a male mold member and a female mold member which can form a molding cavity corresponding to the thick part and the general part wherein each of the mold members has a pressure-reducing hole, the process comprising the following steps (1)-(4):
(1) heating step comprising softening the foamed thermoplastic resin sheet by heating it while claming and fixing its periphery with a clip;
(2) first sucking and shaping step comprising forming a molding cavity through fitting together the male and female mold members located in opposite sides of the foamed thermoplastic resin sheet by moving at least one of the mold members, and sucking through the pressure-reducing hole dug in one mold member (a first sucking mold member) of the male and female mold members, thereby shaping the foamed thermoplastic resin sheet held in the molding cavity into the shape of the first sucking mold member;
(3) second sucking and shaping comprising sucking through the pressure-reducing hole dug in the other mold member (a second sucking mold member) in parallel with or after the first sucking and shaping step, thereby forming a foamed thermoplastic resin molding; and
(4) releasing step comprising removing the foamed thermoplastic resin molding after cooling.
The aforementioned production process can effectively produce foamed thermoplastic resin moldings having a general and thick parts from a foamed thermoplastic resin sheet, as a raw material resin sheet.
In the aforementioned production process, it is preferable that the process further comprises
(5) expansion step comprising generating pressure difference between both sides of the softened foamed thermoplastic resin sheet, thereby causing the foamed thermoplastic resin sheet to project toward the lower pressure side, this step following the heating step
wherein the first sucking and shaping step is
(2xe2x80x2) first sucking and shaping step comprising locating the male and female mold members in opposite sides of the foamed thermoplastic resin sheet so that the male mold member is in the lower pressure side, forming a molding cavity through fitting together the male and female mold members by moving the male mold member with in contact with the softened foamed thermoplastic resin sheet from its projecting side, and sucking through the pressure-reducing hole dug in one mold member (a first sucking mold member) of the male and female mold members, thereby shaping the foamed thermoplastic resin sheet held in the molding cavity into the shape of the first sucking mold member.
When expansion step (5) is added, the foamed thermoplastic resin sheet heated and softened, which is the raw material resin sheet to be molded, is uniformly expanded and stretched through the generation of differential pressure between its opposite sides, and then first sucking and shaping step (2xe2x80x2) is executed. In this step, after fitting the male mold members having a product sharp by moving it to the male mold member side with sequentally in contact with the foamed thermoplastic resin sheet entirely heated and softened from the end portion of the mold from its projecting side, the first sucking and shaping step is executed.
In the case where a polyolefin-based foamed resin sheet is used as the foamed thermoplastic resin sheet, which is the raw material resin sheet to be molded, since it has low melt strength as distinct from amorphous foamed polystyrene resin sheets, great local elongation deformations easily occur in the vicinity of the region where the sheet touches the mold first in the production of a molding with a high drawing ratio, that is, a high ratio of the diameter of its opening to the drawing depth. This easily leads to the occurrence of defects such as the breakage of cells in the foam and the breakage of the sheet.
By the addition of such expansion step, the molding is conducted after preliminarily stretching to some extent the heated polyolefin-based foamed resin sheet with low strength. This makes deeply-drawn foamed thermoplastic resin moldings comprising a thick and general parts possible to be produced without causing such local elongation of the sheet.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
Throughout this specification and the claims which follow, unless the context requires otherwise, the word xe2x80x9ccomprisexe2x80x9d, and variations such as xe2x80x9ccomprisesxe2x80x9d and xe2x80x9ccomprisingxe2x80x9d, will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integer or step.